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  <title>Limitations</title>

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</div><hr /><div id="sdodasrel.limitations" class="chapter">
 <h1>Limitations</h1>

 <p class="para">
  There are the following limitations in the current release of the 
  Relational DAS:
  <ul class="itemizedlist">
   <li class="listitem">
    <p class="para">
     No support for nulls. There is no support for SQL NULL type. 
     It is not legal to assign PHP NULL to a data object property
     and the Relational DAS will not write that back as a NULL to the 
     database. If nulls are found in the database on a query, 
     the property will remain unset.
    </p>
   </li>
   <li class="listitem">
    <p class="para">
     Only two types of SDO relationship. The metadata described below 
     allows the Relational DAS to model just two types of SDO relationship:
     multi-valued containment relationships and single-valued 
     non-containment references. In SDO, whether a property describes a 
     single- or multi-valued relationship, and whether it is containment 
     or non-containment, are independent. The full range of possibilities 
     that SDO allows cannot all be defined. There may be relationships 
     that it would be useful to model but which the current implementation
     cannot manage. 
     One example is a single-valued containment relationship.
    </p>
   </li>
   <li class="listitem">
    <p class="para">
     No support for the full range of SDO data types. The Relational 
     DAS defines all primitive properties in the SDO model as being 
     of type string. SDO defines a richer set of types containing various
     integer, float, boolean and data and time types. String is adequate 
     for the purposes of the Relational DAS since the combination of PHP,
     PDO and the database will ensure that values passed as strings 
     will be converted to the proper type before being put in the database.
     This does affect some scenarios in which the Relational DAS has 
     to work with a data graph that has come from or will go to a 
     different DAS.
    </p>
   </li>
   <li class="listitem">
    <p class="para">
     Only one foreign key per table. 
     The metadata only provides the means to specify one foreign key 
     per table. This foreign key may be mapped to one of the two types of 
     SDO relationship supported. Obviously there are some scenarios 
     that cannot be described under this limitation - it is not possible 
     to have two non-containment references from one table to another 
     for example.
    </p>
   </li>
  </ul>
 </p>
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